Sulphur Recovery Plants are designed to remove H2S from H2S-containing acid gases from Amine Regeneration Systems and from Sour Water Strippers producing sulphur, a non toxic product which can be stored and sold in liquid or in solid form to different users for several different industrial applications. The acid gases from Amine Regeneration Systems and Sour Water Strippers, containing a variable amount of H2S, are treated in a Sulphur Recovery Unit (SRU), generally based on the modified Claus process, for bulk sulphur recovery and subsequently in a Tail Gas Treatment (TGT) section for deep sulphur recovery. Other impurities contained in the sour gases, including ammonia and hydrocarbons, are destroyed in the Claus section.
The modified Claus process by itself recovers about 94÷96% (2 catalytic stages) or 95÷98% (3 stages) of the sulphur in the feedstock. A further treatment of the Claus tail gas is therefore necessary when a higher Sulphur Recovery Efficiency (SRE) is required.
The modified Claus process comprises a sub-stoichiometric combustion of the acid gas stream in a thermal reactor (thermal stage) followed by catalytic conversion in the Claus reactors (catalytic stage). In the Claus section one-third of the total H2S is oxidized to SO2, which reacts with the remaining H2S to form sulphur and water according to the following reactions:H2S+1.5O2→H2O+SO2 (oxidation reaction)  (1)2H2S+SO2↔1.5S2+2H2O (Claus reaction)  (2)3H2S+1.5O2↔3H2O+1.5S2 (overall reaction)  (3)
The goal of the process is to drive the overall reaction to near completion. In the Claus thermal reactor, the H2S contained in the acid gas is burnt with air (or with oxygen-enriched air in some specific cases) in a specific burner and only one-third of the total H2S is oxidized to SO2, while the remaining two-third is not reacted. The total air amount is the one exactly sufficient to oxidize one-third of the total H2S and to completely oxidize all hydrocarbons and ammonia contained in the feedstock; the molar ratio H2S/O2 in the feedstock is therefore about 2:1 in order to get a ratio H2S/SO2 in the Claus tail gas of exactly, or as close as possible to, 2:1, which is the stoichiometric ratio for the Claus reaction, so maximizing Sulphur Recovery Efficiency. During acid gas combustion, a small part of the H2S (typically 5÷7%) is dissociated to hydrogen and sulphur as per following reaction:H2S↔H2+0.5S2 (dissociation or cracking reaction)  (4)
According to Clark et al., Alberta Sulphur Research Ltd. (ASRL), hydrogen formation also happens according to the following reaction:4H2S+O2↔2H2+2H2O+2S2 (H2 formation reaction)  (5)
Several side reactions are also involved, leading to the destruction of ammonia and hydrocarbons and to the formation of carbonyl sulphide COS and carbon disulphide CS2. In order to complete the Claus reactions, a suitable residence time is necessary at high temperature in the thermal reactor.
The Claus thermal reactor is typically followed by a waste heat boiler where furnace effluent is cooled down to about 300° C. and heat is recovered by raising high pressure steam and by a sulphur condenser where process gas is cooled down to sulphur dew point by raising low pressure steam and liquid sulphur is separated.
The Claus thermal stage is generally followed by two or three catalytic stages, each one composed by a gas reheater to bring the gas to the optimal reaction temperature, a catalytic reactor where the Claus reaction takes place and a sulphur condenser where gas is cooled and liquid sulphur is condensed and separated. The Claus reaction is an exothermic equilibrium reaction thermodynamically enhanced by low temperatures. The first Claus catalytic reactor is partly filled with a Claus catalyst (Alumina based) to enhance the Claus reaction and partly filled with a specific high conversion catalyst (Titania based) to enhance the hydrolysis of COS and CS2. The second and third Claus catalytic reactors, if any, are generally filled with Claus catalyst (Alumina based) to enhance Claus reaction.
In order to satisfy the >99% sulphur recovery efficiency normally required for a Sulphur Recovery Plant, the Claus section is generally followed by a Tail Gas Treatment section. Several different alternative processes have been proposed over the years to boost Sulphur Recovery Efficiency, like the SCOT method by Shell Oil Company, the RAR process by TKT, the CBA process by AMOCO, the CLINSULF/DEGSULF method by Linde Actiengesellschaft or the BSR Selectox process by UOP. In the traditional reductive Tail Gas Treatment section, the process gas from a Claus section is preheated and combined with hydrogen from an external source prior to being fed to a hydrogenation reactor, where all sulphur compounds are converted to H2S over a specific reduction catalyst (Co and Mo oxides based), which performs both the hydrogenation and the hydrolysis functions. The reactor effluent is cooled down in the quench tower by means of circulating steam condensate. The H2S produced in the hydrogenation reactor is recovered in an amine absorber with a specific amine aqueous solution and recycled to the Claus section from the top of an amine regenerator, where the enriched solution is stripped.
The tail gas from the amine absorber is sent to a thermal incinerator for the oxidation of residual H2S and other sulphur compounds, such as COS and CS2, to SO2 prior to disposal to the atmosphere via a dedicated stack.
The main drawbacks of traditional Claus Plant are the need for large and expensive equipment against very low sulphur economic value, continuous emissions of SOx (SO2 and SO3), CO, CO2, NOx plus traces of H2S into the atmosphere, and continuous import of hydrogen from the network, for process gas reduction in the TGT section.
In some Plants, where hydrogen is not available, for example in gas fields, the reducing gas mixture is generated in a reducing gas generator by sub-stoichiometric fuel gas combustion. The main drawback of such alternative configuration is the 10÷15% higher process gas flow rate and subsequent larger equipment size due to substantial quantities of inerts coming from in-line fuel gas combustion (mainly nitrogen from air and water and carbon dioxide from combustion).
Some alternative processes have been proposed over the years, which are addressed to thermal or catalytic partial oxidation of H2S.
U.S. Pat. No. 4,481,181 by GA Technologies Inc. discloses a process for removing sulphur and recovering hydrogen from a H2S-containing gas stream coupling thermal partial oxidation of H2S to sulphur and water and thermal dissociation of H2S to hydrogen and sulphur in the same reaction zone, preceded by feedstock heating section and followed by a cooling zone and by a sulphur condenser, using pure oxygen and a substantial proportion of nitrogen with a H2S/O2 ratio in the feedstock between 10:1 and 25:1. The main goal of this patent is to thermally decompose by partial oxidation and dissociation hydrogen sulphide into sulphur and hydrogen.
WO2010/036941 by Chevron U.S.A. Inc. and Drexel University discloses a method for performing H2S thermal dissociation at temperature below 1600° C. based on H and SH radicals, in one embodiment over a suitable plasma catalyst.
Furthermore, Italian Patent 1 203 898 by Siirtec-Nigi discloses a process called HCR based on the operation of the traditional Claus thermal reactor at a slightly higher H2S/O2 ratio in the feedstock in order to keep a H2S/SO2 ratio in the Claus tail gas significantly higher than 2:1. The main goal of this process is to boost hydrogen production in thermal reactor and to avoid hydrogen import in the TGT section. Also with such a process, Sulphur Recovery Plant emissions are not avoided.
From the above discussion, it is evident that several efforts have been made in the past, trying to propose a valid alternative to traditional Claus Plant. In particular, some processes which have been proposed over the years are based on the thermal or catalytic partial oxidation of H2S, while some other processes are focused on the thermal or catalytic cracking of H2S. None of the proposed processes is conceived and arranged to perform H2S conversion to hydrogen and sulphur aiming at favoring both reactions at the same time.
It would be desired to reduce, and preferably avoid, emissions into the atmosphere. It would also be desired to reduce, and preferably avoid, the importation of hydrogen into the process. Particularly, it would be desired to generate hydrogen, and to optimize the export of hydrogen from the process, yet with concurrent production of sulphur.